As a coding scheme which can compress speech, music, or the like at a low bit rate and with high sound quality, there has been proposed a coding scheme which combines a CELP (Code Excited Linear Prediction) coding scheme suitable for a speech signal with a transform coding scheme suitable for a music signal in a layered structure (see, for example, Non-Patent Literature (hereinafter, referred to as “NPL”) 1). Hereinafter, a speech signal and a music signal may be collectively referred to as a sound signal.
In this coding scheme, an encoding apparatus first encodes an input signal using the CELP coding scheme to generate CELP encoded data. The encoding apparatus then performs transform coding on a residual spectrum obtained by converting a residual signal between the input signal and a CELP decoded signal (result of decoding the CELP encoded data) (hereinafter, referred to as a CELP residual signal) into a frequency domain, thereby providing high sound quality. As the transform coding scheme, there has been proposed a scheme which generates pulses at frequencies having high residual spectrum energy and encodes information of the pulses (see NPL 1).
However, although the CELP coding scheme is suitable for encoding a speech signal, the CELP coding scheme may degrade the sound quality of a music signal because the coding model is different for a music signal. Accordingly, when a music signal is encoded using the CELP coding scheme, there is a problem in that it is difficult to improve the sound quality even when the CELP residual signal (residual spectrum) is encoded using the transform coding scheme because of a larger component of the CELP residual signal.
To solve the above problem, there has been proposed a coding scheme which provides high sound quality by performing transform coding on a residual spectrum calculated using a result of suppressing an amplitude of a frequency component of a CELP decoded signal (hereinafter, referred to as a CELP component) (hereinafter, referred to as a CELP component suppressing method) (see, for example, Patent Literature (hereinafter referred to as “PTL”) 1 and NPL 1 (section 6.11.6.2)).
In the CELP component suppressing method disclosed in NPL 1, when a sampling frequency of an input signal is 16 kHz, the amplitude of the CELP component is suppressed (hereinafter, referred to as CELP suppression) only in an intermediate band between 0.8 kHz and 5.5 kHz. In NPL 1, an encoding apparatus does not perform transform coding on the CELP residual signal alone, but applies another transform coding scheme in a lower layer prior to the transform coding, thereby reducing the CELP residual signal (see, for example, NPL 1 (section 6.11.6.1)). Therefore, the encoding apparatus does not perform CELP suppression on the frequency component encoded by another transform coding scheme even in the intermediate band and passes the transform encoded data in the lower layer to a higher layer and determines whether each frequency component has a value other than 0. For a frequency component having a value other than 0, the encoding apparatus determines that transform coding has been performed in the lower layer and does not perform CELP suppression. Meanwhile, for a frequency component having a value of 0, the encoding apparatus determines that transform coding has not been performed in the lower layer and performs CELP suppressing processing. CELP suppressing coefficients indicating the degrees (levels) of CELP suppression are uniform in frequencies in the intermediate band other than the frequencies for which CELP suppression is not performed.
The encoding apparatus performs CELP suppression by multiplying the CELP component (CELP decoded signal) by a CELP suppressing coefficient stored in a CELP suppressing coefficient code book prior to the transform coding and then obtains a residual spectrum between the input signal and the CELP decoded signal (CELP decoded signal that has been subjected to the CELP suppression) and performs transform coding on the residual spectrum.
Meanwhile, a decoding apparatus suppresses the CELP component of the CELP decoded signal using a CELP suppressing coefficient transmitted from the encoding apparatus and adds a transform coding decoded signal to the CELP decoded signal in which the CELP component has been suppressed. By this means, the decoding apparatus can obtain a decoded signal with less degradation in sound quality due to CELP coding when performing coding in which CELP coding and transform coding are combined in a layered structure.